1. Field of the Invention
The present invention relates to a nitride-based semiconductor element and a method of forming a nitride-based semiconductor, and more specifically, it relates to a nitride-based semiconductor element containing a nitride-based semiconductor formed by epitaxial lateral overgrowth and a method of forming a nitride-based semiconductor.
2. Description of the Prior Art
A technique of growing a nitride-based semiconductor on an underlayer is known in general. For example, a crystal of GaN, which is one of nitride-based semiconductors lattice-matching with only a small number of types of substrates, is grown on a substrate such as a sapphire substrate. In relation to this, generally known is a technique of inserting a buffer layer grown at a low temperature between the substrate and a GaN layer for growing GaN in excellent crystallinity with a small number of crystal defects.
Even if the aforementioned low-temperature buffer layer is employed, however, the density of reducible defects is limited and it is difficult to reduce the number of dislocations. To this end, a technique of reducing the number of dislocations by epitaxial lateral overgrowth (ELOG) of GaN is proposed in general. This epitaxial lateral overgrowth is disclosed in Journal of Oyo Denshi Bussei Bunkakai, Vol. 4 (1998), pp. 53 to 58 and 210 to 215, for example.
FIG. 29 is a sectional view for illustrating a conventional method of forming a nitride-based semiconductor by epitaxial lateral overgrowth. Referring to FIG. 29, a low-temperature buffer layer 102 is first formed on a sapphire substrate 101, and thereafter a GaN layer 103 for serving as an underlayer is grown on the low-temperature buffer layer 102 in the conventional method of forming a nitride-based semiconductor by epitaxial lateral overgrowth.
Then, striped (elongated) mask layers 104 of SiO2 or the like are formed on prescribed regions of the GaN layer 103. The mask layers 104 are employed as selective growth masks for epitaxially laterally overgrowing a GaN layer 105 from the GaN layer 103 serving as an underlayer, so that the GaN layer 105 is vertically (upwardly) grown from exposed portions of the GaN layer 103 and thereafter laterally grown on the mask layers 104. Dislocations extending in the c-axis direction are laterally bent due to this lateral overgrowth, not to reach a portion around the upper surface of the GaN layer 105. Thus, the number of dislocations reaching the flat upper surface of the finally formed GaN layer 105 is remarkably reduced as compared with the GaN layer 103 forming the underlayer.
In the conventional method of forming a nitride-based semiconductor by epitaxial lateral overgrowth shown in FIG. 29, however, the c-axis of the overgrowth region of GaN layer 105 is disadvantageously inclined from the normal direction of the substrate although the number of dislocations resulting from epitaxial lateral overgrowth can be reduced in the GaN layer 105. In other words, the growth layer of the GaN layer 105 laterally grown on the mask layers 104 is strained due to stress applied thereto. Therefore, the c-axis perpendicular to the sapphire substrate 101 is disadvantageously inclined by about 2xc2x0 at the maximum as shown by arrows in FIG. 29. When the c-axis is displaced, crystallinity is deteriorated to result in inferior element characteristics.
In order to suppress such inclination of the c-axis, therefore, a method shown in FIG. 30 is proposed in general. Referring to FIG. 30, a GaN layer 113 serving as an underlayer is formed on a sapphire substrate 111 through a low-temperature buffer layer 112 in this proposed method. Recess portions are formed on the surface of the GaN layer 113 for thereafter forming mask layers 114 having recess portions 114a of SiO2 or the like in recess portions 113a of the surface. The mask layers 114 are employed as selective growth masks for epitaxially laterally overgrowing a GaN layer 115 on projection portions of the GaN layer 113 serving as an underlayer. In this case, voids 120 are defined between the mask layers 114 and the epitaxially laterally overgrown GaN layer 115, thereby reducing the contact areas between the GaN layer 115 and the mask layers 114 when the GaN layer 115 is laterally grown on the mask layers 114. Thus, stress is hardly applied to the GaN layer 115 laterally grown on the mask layers 114, and hence strain of the GaN layer 115 is relaxed. Consequently, inclination of the c-axis can be relaxed as compared with the prior art shown in FIG. 29, as shown by arrows in FIG. 30.
In the conventional proposed method shown in FIG. 30, however, it is necessary to etch the GaN layer 113 serving as an underlayer for forming the recess portions thereof. In this case, a long time is required for etching the GaN layer 113, disadvantageously leading to a long process time.
In the conventional proposed method shown in FIG. 30, most dislocations 116 are bent in intermediate portions during lateral growth of the GaN layer 115 not to reach the surface, as shown in FIG. 31. However, some of dislocations 116 are not bent but reach the surface as such. In the conventional proposed method, therefore, it is difficult to further reduce the number of dislocations.
An object of the present invention is to provide a nitride-based semiconductor element having excellent device characteristics by obtaining a nitride-based semiconductor layer having excellent crystallinity without performing a long-time etching process.
Another object of the present invention is to reduce the number of dislocations in an epitaxially laterally overgrown nitride-based semiconductor layer in the aforementioned nitride-based semiconductor element.
Still another object of the present invention is to provide a method of forming a nitride-based semiconductor capable of readily forming a nitride-based semiconductor layer having excellent crystallinity without performing a long-time etching process.
A further object of the present invention is to more effectively reduce the number of dislocations reaching the surface of the nitride-based semiconductor in the aforementioned method of forming a nitride-based semiconductor.
A nitride-based semiconductor element according to a first aspect of the present invention comprises a mask layer, having a recess portion on the upper surface of the mask layer, formed on a substantially flat upper surface of an underlayer to partially expose the upper surface of the underlayer, a nitride-based semiconductor layer formed on the exposed part of the underlayer and the mask layer while forming a void on the recess portions of the mask layer, and a nitride-based semiconductor element layer, formed on the nitride-based semiconductor layer, having an element region.
In the nitride-based semiconductor element according to the first aspect, the mask layer having a recess portion is formed on the substantially flat upper surface of the underlayer as hereinabove described, whereby the nitride-based semiconductor layer is formed with the void on the recess portion of the mask layer when laterally grown on the mask layer. This void relaxes strain of the laterally grown nitride-based semiconductor layer, whereby inclination of the c-axis (crystal axis) of the nitride-based semiconductor layer can be suppressed. Further, the underlayer is formed in a substantially flat shape, whereby no etching may be performed over a long time for forming recess portions on the surface of the underlayer consisting of a nitride-based semiconductor or the like, dissimilarly to the prior art. Therefore, inclination of the c-axis (crystal axis) of the nitride-based semiconductor layer can be suppressed without performing an etching process for a long time. Consequently, a nitride-based semiconductor layer having excellent crystallinity can be obtained without performing a long-time etching process. When the nitride-based semiconductor element layer having an element region is grown on the nitride-based semiconductor layer having excellent crystallinity, a nitride-based semiconductor element having excellent element characteristics can be obtained.
In the aforementioned nitride-based semiconductor element according to the first aspect, the recess portion of the mask layer preferably includes a dent provided on at least part of the upper surface of the mask layer. According to this structure, the void can be readily formed on the mask layer due to the dent when the nitride-based semiconductor layer is laterally grown on the mask layer.
In the aforementioned nitride-based semiconductor element according to the first aspect, the recess portion of the mask layer preferably includes a concavely curved upper surface of the mask layer. According to this structure, the void can be readily formed on the mask layer due to the concavely curved upper surface of the mask layer. In this case, the mask layer preferably has an overhanging shape, and the upper surface of the overhanging mask layer is preferably concavely curved. In this case, further, the overhanging mask layer preferably includes a first insulator film formed on the underlayer and a second insulator film, formed on the first insulator film, having a smaller etching rate than the first insulator film. According to this structure, the overhanging mask layer having a laterally protruding second insulator film can be readily obtained by forming the first and second insulator films and thereafter etching the second and first insulator films.
In the aforementioned nitride-based semiconductor element according to the first aspect, the underlayer preferably includes a substrate, and the mask layer is preferably formed to be in contact with the upper surface of the substrate. According to this structure, no underlayer may be formed on the substrate, whereby the fabrication process can be simplified. Further, the total thickness can be reduced due to absence of the underlayer.
A nitride-based semiconductor element according to a second aspect of the present invention comprises a mask layer, having a recess portion on the upper surface of the mask layer, formed on projection portions of an underlayer having the projection portions on an upper surface to partially expose the upper surface of the underlayer, a nitride-based semiconductor layer formed on the exposed part of the underlayer and the mask layer while forming a void on the recess portion of the mask layer, and a nitride-based semiconductor element layer, formed on the nitride-based semiconductor layer, having an element region.
In the nitride-based semiconductor element according to the second aspect, the mask layer having a recess portion is formed on the upper surface of the underlayer as hereinabove described, whereby the nitride-based semiconductor layer is formed with the void on the recess portion of the mask layer when laterally grown on the mask layer. This void relaxes strain of the laterally grown nitride-based semiconductor layer, whereby inclination of the c-axis (crystal axis) of the nitride-based semiconductor layer can be suppressed. When the recess portions are formed on the underlayer and the mask layer is formed on projection portions thereof to have an overhanging shape, raw material is hardly supplied to a portion located under the overhang of the mask layer, to result in difference in growth rate between the portion located under the overhang and the remaining portions. Thus, lateral growth is dominant in the portion located under the overhang having a low growth rate from an extremely initial stage, whereby dislocations start to laterally bend from the initial stage. Consequently, it is possible to more effectively prevent the dislocations from reaching the surface of the nitride-based semiconductor layer. Therefore, inclination of the c-axis (crystal axis) of the nitride-based semiconductor layer can be suppressed while further reducing the number of dislocations. Consequently, a nitride-based semiconductor layer having more excellent crystallinity can be obtained. When the nitride-based semiconductor element layer having an element region is grown on the nitride-based semiconductor layer having more excellent crystallinity, a nitride-based semiconductor element having more excellent element characteristics can be obtained.
In the aforementioned nitride-based semiconductor element according to the second aspect, the recess portion of the mask layer preferably includes a dent provided on at least part of the upper surface of the mask layer. According to this structure, the void can be readily formed on the mask layer due to the dent when the nitride-based semiconductor layer is laterally grown on the mask layer.
In the aforementioned nitride-based semiconductor element according to the second aspect, the recess portion of the mask layer preferably includes a concavely curved upper surface of the mask layer. According to this structure, the void can be readily formed on the mask layer due to the concavely curved surface of the mask layer. In this case, the mask layer preferably has an overhanging shape, and the upper surface of the overhanging mask layer is preferably concavely curved.
In the aforementioned nitride-based semiconductor element according to the second aspect, the underlayer preferably includes a substrate, and the mask layer is preferably formed to be in contact with the upper surface of the substrate. According to this structure, no underlayer may be formed on the substrate, whereby the fabrication process can be simplified. Further, the total thickness can be reduced due to absence of the underlayer.
A method of forming a nitride-based semiconductor according to a third aspect of the present invention comprises steps of forming a mask layer, having a recess portion on the upper surface of the mask layer on a substantially flat upper surface of an underlayer to partially expose the upper surface of the underlayer, and growing a nitride-based semiconductor layer on the exposed part of the underlayer and the mask layer.
In the method of forming a nitride-based semiconductor according to the third aspect, the mask layer having a recess portion is formed as hereinabove described, whereby the nitride-based semiconductor layer is formed with the void on the recess portion of the mask layer when laterally grown on the mask layer. The void relaxes strain of the laterally grown nitride-based semiconductor layer, whereby inclination of the c-axis (crystal axis) of the nitride-based semiconductor layer can be suppressed. The underlayer is formed in a substantially flat shape, whereby no etching may be performed over a long time for forming recess portions on the underlayer consisting of a nitride-based semiconductor or the like, dissimilarly to the prior art. Thus, inclination of the c-axis (crystal axis) of the nitride-based semiconductor layer can be suppressed without performing an etching process for a long time. Consequently, a nitride-based semiconductor layer having excellent crystallinity can be readily formed without performing a long-time etching process.
In the aforementioned method of forming a nitride-based semiconductor according to the third aspect, the step of forming the mask layer preferably includes a step of forming the mask layer on a prescribed region of the underlayer and thereafter partially etching the upper surface of the mask layer thereby forming the recess portion on the upper surface of the mask layer. According to this structure, the mask layer having the recess portion on the upper surface can be readily formed.
In the aforementioned method of forming a nitride-based semiconductor according to the third aspect, the step of forming the mask layer preferably includes steps of forming a first mask material layer on part of a region of the underlayer formed with the mask layer, forming a second mask material layer to cover the first mask material layer and the underlayer, and etching the second mask material layer while leaving the first mask material layer thereby forming the mask layer consisting of the first mask material layer and the second mask material layer and having the recess portion on the upper surface. According to this structure, the mask layer having the recess portion on the upper surface can be readily formed.
In the aforementioned method of forming a nitride-based semiconductor according to the third aspect, the underlayer preferably includes a substrate, and the step of forming the mask layer preferably includes a step of forming the mask layer to be in contact with the upper surface of the substrate. According to this structure, no underlayer may be formed on the substrate, whereby the fabrication process can be simplified. Further, the total thickness can be reduced due to absence of the underlayer.
The aforementioned method of forming a nitride-based semiconductor according to the third aspect preferably further comprises a step of growing a nitride-based semiconductor element layer having an element region on the nitride-based semiconductor layer. According to this structure, the nitride-based semiconductor element layer having an element region is grown on the nitride-based semiconductor layer having excellent crystallinity, whereby a nitride-based semiconductor element having excellent element characteristics can be readily formed.
A method of forming a nitride-based semiconductor according to a fourth aspect of the present invention comprises steps of forming a mask layer, having a recess portion on the upper surface of the mask layer on projection portions of an underlayer having the projection portions on upper surface for partially exposing the surface of the underlayer, and growing a nitride-based semiconductor layer on the exposed part of the underlayer and the mask layer.
In the method of forming a nitride-based semiconductor according to the fourth aspect, the mask layer having a recess portion is formed on the upper surface of the underlayer, whereby the nitride-based semiconductor layer is formed while a void remaining on the recess portion of the mask layer when laterally grown on the mask layer. This void relaxes strain of the laterally grown nitride-based semiconductor layer, whereby inclination of the c-axis (crystal axis) of the nitride-based semiconductor layer can be suppressed. When the projection portion is formed on the underlayer and the mask layer is formed on the projection portions thereof to have an overhanging shape, raw material is hardly supplied to a portion located under the overhang of the mask layer, inducing difference in growth rate between the portion located under the overhang and the remaining portions. Thus, lateral growth is dominant in the portion located under the overhang having a low growth rate from an extremely initial stage, whereby dislocations start to laterally bend from the initial stage. Consequently, it is possible to more effectively prevent the dislocations from reaching the surface of the nitride-based semiconductor layer. Therefore, inclination of the c-axis (crystal axis) of the nitride-based semiconductor layer can be suppressed while further reducing the number of dislocations. Consequently, a nitride-based semiconductor layer having more excellent crystallinity can be obtained.
In the aforementioned method of forming a nitride-based semiconductor according to the fourth aspect, the underlayer preferably includes a substrate, and the step of forming the mask layer preferably includes a step of forming the mask layer to be in contact with the upper surface of the substrate. According to this structure, no underlayer may be formed on the substrate, whereby the fabrication process can be simplified. Further, the total thickness can be reduced due to absence of the underlayer.
The aforementioned method of forming a nitride-based semiconductor according to the fourth aspect preferably further comprises a step of growing a nitride-based semiconductor element layer having an element region on the nitride-based semiconductor layer. According to this structure, the nitride-based semiconductor element layer having an element region is grown on the nitride-based semiconductor layer having excellent crystallinity, whereby a nitride-based semiconductor element having excellent element characteristics can be readily formed.
A method of forming a nitride-based semiconductor according to a fifth aspect of the present invention comprises steps of forming a mask layer having an overhanging shape on a substantially flat upper surface of an underlayer to expose part of the flat upper surface of the underlayer, and growing a nitride-based semiconductor layer on the exposed part of the underlayer and the mask layer.
In the method of forming a nitride-based semiconductor according to the fifth aspect, the overhanging mask layer is formed as hereinabove described, whereby the nitride-based semiconductor layer applies upward force to the protrusion of the overhanging mask layer in the process of growth, for curving the upper surface of the overhanging mask layer and defining a dent. Thus, the nitride-based semiconductor layer is formed while forming a void on the upper surface of the curved dent of the mask layer. This void relaxes strain of the laterally grown nitride-based semiconductor layer, whereby inclination of the c-axis (crystal axis) of the nitride-based semiconductor layer can be suppressed. In this case, the curved dent of the mask layer reduces the contact area between the upper surface of the mask layer and the growth layer, whereby inclination of the c-axis can be reduced to substantially 0xc2x0. Consequently, a nitride-based semiconductor layer having more excellent crystallinity can be readily formed. When the underlayer is formed in a substantially flat shape, no etching may be performed over a long time for forming recess portions on the underlayer consisting of a nitride-based semiconductor or the like, dissimilarly to the prior art. Therefore, inclination of the c-axis (crystal axis) of the nitride-based semiconductor layer can be suppressed without performing an etching process over a long time. Consequently, a nitride-based semiconductor layer having more excellent crystallinity can be readily formed without performing a long-time etching process.
According to the fifth aspect, the step of growing the nitride-based semiconductor layer preferably includes a step of growing the nitride-based semiconductor layer from under the mask layer having an overhanging shape and applying force from under the overhang of the mask layer thereby curving the upper surface of the overhanging mask layer. According to this structure, the upper surface is curved to define a dent, whereby the nitride-based semiconductor layer can be readily formed while forming a void on the dent of the mask layer.
According to the fifth aspect, the step of forming the mask layer preferably includes steps of forming a first mask material layer on the underlayer while forming a second mask material layer having a smaller etching rate than the first mask material layer on the first mask material layer, and etching the first mask material layer and the second mask material layer thereby forming the overhanging mask layer having the first mask material layer and the second mask material layer. According to this structure, the overhanging mask layer can be readily formed.
In the aforementioned method of forming a nitride-based semiconductor according to the fifth aspect, the underlayer preferably includes a substrate, and the step of forming the mask layer preferably includes a step of forming the mask layer to be in contact with the upper surface of the substrate. According to this structure, no underlayer may be formed on the substrate, whereby the fabrication process can be simplified. Further, the total thickness can be reduced due to absence of the underlayer.
The aforementioned method of forming a nitride-based semiconductor according to the fifth aspect preferably further comprises a step of growing a nitride-based semiconductor element layer having an element region on the nitride-based semiconductor layer. According to this structure, the nitride-based semiconductor element layer having an element region is grown on the nitride-based semiconductor layer having excellent crystallinity, whereby a nitride-based semiconductor element having excellent element characteristics can be readily formed.
A method of forming a nitride-based semiconductor according to a sixth aspect of the present invention comprises steps of forming a mask layer, having an overhanging shape on projection portions of an upper surface of an underlayer having the projection portions to expose part of the upper surface of the underlayer, and growing a nitride-based semiconductor layer on the exposed part of the underlayer and the mask layer.
In the method of forming a nitride-based semiconductor according to the sixth aspect, the overhanging mask layer is formed as hereinabove described, whereby the nitride-based semiconductor layer applies upward force to the protrusion of the overhanging mask layer in the process of growth, for curving the upper surface of the overhanging mask layer and forming concavely curved surface. Thus, the nitride-based semiconductor layer is formed while forming a void on the upper surface of the curved dent of the mask layer. This void relaxes strain of the laterally grown nitride-based semiconductor layer, whereby inclination of the c-axis (crystal axis) of the nitride-based semiconductor layer can be suppressed. In this case, the curved dent of the mask layer reduces the contact area between the upper surface of the mask layer and the growth layer, whereby inclination of the c-axis can be reduced to substantially 0xc2x0. Consequently, a nitride-based semiconductor layer having more excellent crystallinity can be readily formed. When the underlayer is formed on projection portions and the mask layer is formed on projection portions thereof to have an overhanging shape, raw material is hardly supplied to a portion located under the overhang of the mask layer, to result in difference in growth rate between the portion located under the overhang and the remaining portions. Thus, lateral growth is dominant in the portion located under the overhang having a low growth rate from an extremely initial stage, whereby dislocations start to laterally bend from the initial stage. Consequently, it is possible to more effectively prevent the dislocations from reaching the surface of the nitride-based semiconductor layer. Therefore, inclination of the c-axis (crystal axis) of the nitride-based semiconductor layer can be suppressed while further reducing the number of dislocations. Consequently, a nitride-based semiconductor layer having more excellent crystallinity can be obtained.
According to the aforementioned sixth aspect, the step of growing the nitride-based semiconductor layer preferably includes a step of growing the nitride-based semiconductor layer from under the mask layer having an overhanging shape and applying force from under the overhang of the mask layer thereby curving the upper surface of the overhanging mask layer. According to this structure, the upper surface is curved to define a dent, whereby the nitride-based semiconductor layer can be readily formed while forming a void on the dent of the mask layer.
In the aforementioned method of forming a nitride-based semiconductor according to the sixth aspect, the underlayer preferably includes a substrate, and the step of forming the mask layer preferably includes a step of forming the mask layer to be in contact with the upper surface of the substrate. According to this structure, no underlayer may be formed on the substrate, whereby the fabrication process can be simplified. Further, the total thickness can be reduced due to absence of the underlayer.
The aforementioned method of forming a nitride-based semiconductor according to the sixth aspect preferably further comprises a step of growing a nitride-based semiconductor element layer having an element region on the nitride-based semiconductor layer. According to this structure, the nitride-based semiconductor element layer having an element region is grown on the nitride-based semiconductor layer having excellent crystallinity, whereby a nitride-based semiconductor element having excellent element characteristics can be readily formed.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.